Sheet pay-out devices for feeding a sheet from a sheet roll are widely used in various devices and machines that handle flexible sheets. For instance, in a stencil printing device provided with the function of making stencil master plates, a sheet pay-out device is used in a master plate sheet supply unit for feeding a stencil master plate sheet from a stencil master plate sheet roll wound around a central core.
In such a sheet pay-out device, it is often necessary to apply a tension to the sheet so that the sheet may be paid out from the sheet roll without slacking or creasing. Typically, in such a stencil printing device, a frictional resistance is applied to the sheet by pressing a sheet spring upon a flange at one end of the roll or upon a retainer for the flange.
In such a sheet pay-out device, there are two conflicting requirements. One is to avoid slacking or creasing of the sheet. The other is to avoid stretching the sheet. To achieve an acceptable solution, it is necessary to control the tension applied to the sheet.
In particular, because a stencil master plate sheet consisting of a thermoplastic resin film for thermal plate making based on selective perforation is highly flexible and thin, it can easily slack, crease and stretch. If a stencil master plate becomes either slack, creased or stretched, a stable printed image cannot be obtained. It is therefore necessary to appropriately control the tension applied to the stencil master plate sheet.
When a frictional resistance is applied to the rotation of a sheet roll as a back torque for the purpose of applying a tension to the sheet, the tension acting upon the sheet is given by dividing this back torque by the radius of the roll. Therefore, if the frictional resistance acting upon the rotation of the roll is fixed, the tension changes as the sheet is paid out from the roll, and the diameter of the roll diminishes. For instance, if the initial radius of the roll is 45 mm, and the final radius of the roll is 22.5 mm, and if the frictional resistance or the back torque acting upon the rotation of the roll is fixed, the tension acting upon the sheet increases by the factor of two from the initial condition to the final condition.
Therefore, in such a situation, it was conventionally necessary, to the end of ensuring a stable feeding movement of the sheet, to limit the feeding speed of the sheet to a low level, or to provide extra means for preventing the creasing and stretching of the sheet.